How do we prevent a large competitor from just implementing our feature?

If the feature is patentable, then the protection you receive from that patent should be sufficient to either prevent your competitors from implementing the feature, or to sue them when they do. (Features that are easy to detect when implemented are great from the latter perspective, but even for highly embedded functions/algorithms/circuits/etc, there ARE ways to reverse engineer the product and prove infringement.) The easiest way to prove to your investors that you will have this protection is to file for a patent application or at least a provisional patent application. Performing a prior art search would also be very useful to show that your feature is not already patented by others.